The present invention relates generally to noise suppression in radio receivers and, more particularly, to an improved impulse noise blanking arrangement for selectively blanking in either the wideband or narrowband portion of the receiver, dependent upon specific signal conditions.
Impulse noise interference typically includes frequency spectral components which have large amplitude and which are distributed throughout a wide frequency range extending up to several hundred MHz. Thus, impulse noise is capable of interfering with the radio waves within the spectrum which is modulated with desired signal informaton. This interference degrades the receiver performance and appears as an undesirable audio output that may obliterate the desired signal output. Noise blanking circuits are used to alleviate this problem.
Impulse noise blanking circuits for use in communication radio receivers are of course known in the art. Generally, blanker circuits detect noise pulses that are received at the antenna along with transmitted information signals. The detected noise pulses are processed whereby blanking pulses are produced which are employed to blank the receiver for the duration of each blanking pulse, thus preventing the noise pulse from being heard by the listener. Typically the blanker circuits include rate shut-off and level shut-off circuitry to inhibit blanking operation for a predetermined blanking pulse repetition rate and according to noise energy level. Such representative blanking circuits are described and claimed in U.S. Pat. Nos. 3,304,503, 3,699,457, 3,725,674 and 4,334,317 and assigned to the same assignee as the present invention. The U.S. Pat. Nos. 3,699,457, 3,725,674 and 4,334,317 patents teach blanking in the wideband, prefilter stage of the receiver. The U.S. Pat. No. 3,304,503 patent teaches blanking in both the radio frequency (RF) and intermediate frequency (IF) portions of the receiver, but not selectively in either portion. The system of the present invention is an improvement over circuits of this type.
One problem that has arisen in prior systems is that if a noise blanker is used which turns off the radio frequency or wideband stage whenever a noise impulse is present in this stage, an undesirable form of interference known as modulation or blanker splatter may occur. In rapidly turning on and off the radio frequency stage of a receiver, by means of blanking pulses, sidebands are generated through modulation of the undesired signal by the blanking pulses. These sidebands may be very close in frequency to the desired signal and within the passband of the intermediate frequency stages. In such cases the undesired sidebands will not be filtered out and will appear in the receiver output as interference and thus, will degrade the receiver processed signal.
As a noise pulse passes through the frequencyselective stages of a receiver, it is amplified, and stretched in time due to the effects of the filtering. Since the receiver should be turned off during the entire time that the noise pulse is present in the stage of the receiver which is being blanked, blanking in the narrowband, postfilter stages of the receiver requires that the receiver be turned off for an appreciably longer time than if the blanking is accomplished in the wideband, prefilter stages of the receiver. Noticeable audio distortion may occur as a result of the longer blanking pulse which is required when blanking in the narrowband postfilter stages of the receiver. However, narrowband blanking can prevent the impulse noise disturbance from being reproduced by the receiver speaker and degrading the reproduced audio quality when wideband blanking would cause blanker splatter. For signal conditions where blanker splatter results from blanking in the wideband stage of the receiver, blanking in the narrowband stage of the receiver improves the reproduced audio quality while avoiding the generation of blanker splatter.